Pair-gain systems are widely used to expand customer capacity of an existing infrastructure of a PSTN. By using a pair-gain system, a single two-wire channel between a PSTN central office and a neighborhood, which would normally be used to provide telephone service to a single customer in the neighborhood, is usable to provide telephone service to a plurality of customers in the neighborhood.
A pair-gain system generally comprises a central office terminal at or near to a central office of the PSTN and a “remote terminal” in a neighborhood of geographically localized customers serviced by the PSTN. A single two-wire channel connects the central office terminal and the remote terminal.
The central office terminal comprises an analogue front end for each of the customers, a multiplexer and a high-speed modem. The analogue front end for each customer receives analogue signals intended for the customer from a line card in the central office that services the customer. It converts the analogue signals that it receives to digital signals using an A-law or μ-law companding algorithm with 8 bit resolution and a sampling frequency of 8000 samples per second. The digitized signals are transmitted in a DS0 data steam (8000 samples per second at 8 bit resolution) to the multiplexer. The multiplexer multiplexes DS0 data streams from all of the analogue front ends and transmits the multiplexed data streams via a high-speed modem to the remote terminal. The remote terminal, which comprises a line card for each customer, a multiplexer and a high-speed modem, demultiplexes the data streams and routes them to their intended customers. The pair-gain system operates in a similar manner in transmitting voice and data signals upstream, from the customers to the central office.
The pair-gain system provides each customer in the neighborhood with a communication channel that supports voice signal transmission and data signal transmission at rates up to a maximum of 33.6 KBps, which is the maximum transmission rate provided by V.34 voice band modems. This maximum data signal transmission rate supports maximum data transmission rates provided by all V series voice band modems with the exception of maximum transmission rates provided by PCM modems, such as V.90 and V.92 modems, which are higher than the maximum transmission rates of the other V series modems.
ITU-T Recommendation V.90 (09/98) defines specifications for a V.90 digital modem and V.90 analogue modem pair for use on the Public Switched Telephone Network. The recommendation, hereinafter referred to as a V.90 protocol, provides for data signaling rates of up to 56 KBps downstream from the digital modem to the analogue modem and up to 33.6 KBps upstream from the analogue modem to the digital modem. The V.90 analogue modems, hereinafter referred to as “V.90A modems”, are designed according to the protocol to receive analogue data signals from the PSTN at rates up to 56 KBps and transmit data at rates up to 33.6 KBps. The digital V.90 modems, hereinafter referred to as “V.90D modems”, are designed to transmit PCM data at 56 KBps. The design of the V.90 modem pair enables a customer connected to an internet service provider (ISP) that transmits data downstream using a V.90D modem to receive data from the ISP at up to 56 KBps. Upstream data from the user to the ISP, which normally consists of low bandwidth data such as keystrokes and short messages, is comfortably accommodated by the maximum V.90A upstream rate of up to 33.6 KBps.
However, for a V.90A modem to receive V.90 data from a sender, signals that the sender transmits should not undergo an analogue to digital conversion, known as a tandem link, en route to the modem that adds quantization noise to the signals. The analogue to digital conversions that analogue front ends, hereinafter referred to as “conventional front ends”, in conventional pair-gain systems perform on signals that they receive introduce substantial quantization noise into the signals. As a result, a customer with a V.90A modem connected to his or her local central office via a conventional pair-gain system is not able to receive data at the maximum data receive rate that the V.90A modem provides. The customer's V.90A modem will perform a fall back to a V.34 modem protocol (as described in ITU-T V.90 9/98) and limit data receive rates up to a maximum receive rate of 36.6 KBps.